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Hekla eruption 2000

Photographs taken by Gísli Óskarsson (e-mail: gosk@ismennt.is)Courtesy of Gísli and Icelandic National Broadcasting Service (e-mail: tvnews@ruv.is)

Hekla volcano in South Iceland began a new eruption on February 26, 2000 at 18:19 GMT.

A short-term precursory earthquake activity was recorded by the seismic networks of the Science Institute, University of Iceland, and the Iceland Meteorological Office. Small earthquakes were detected by a seismograph near the summit of Hekla beginning at 17:00. This activity gradually increased and the first locatable earthquakes occurred at 17:29. Continuous low-frequency tremor characteristic of Hekla eruptions began at 18:19, and at the same time the eruptive cloud was spotted by observers.

A network of borehole strainmeters operated by the Iceland Meteorological Office also detected precursory strain change associated with magma movements. A warning was issued to the National Civil Defense of Iceland about one hour before the eruption, and the public was alerted about the imminent eruption about 15 minutes before it began, through news on the Iceland national radio.

Initially, a 6-7 km long eruptive fissure opened up along most of the Hekla ridge. A discontinuous curtain of fire emanated from the whole fissure. More than 10 km high ash plume formed within few minutes of the beginning of the eruption, and was carried with light winds towards north. Judged from the amplitude of the eruption tremor on the nearby seismographs the eruption reached peak intensity in the first hour of activity, but then the activity gradually declined.

The maximum thickness of the ash sector, 21 km north of the volcano, was 4-5 cm when measured 7 hours after the onset of the eruption. Most of the ash fell in uninhabited areas in the interior of Iceland, but asmall amount of ash fell in inhabited areas in North Iceland. Ash fall was reported on the Grimsey island off the north cost of Iceland, at a distance of 300 km from Hekla.

On February 27 northerly winds were prevailing and light ash fall occurred in South Iceland. Lava flows down the slopes of Hekla and covers a large part of the SE flank of the Hekla ridge. One lava stream flowed from the eruptive fissure towards the north. In the evening it was slowly advancing at a rate of 1-2 meters per hour. A more active lava stream emanated from three craters near the southern end of the eruptive fissure. This lava stream was several km long and was advancing at a rate of about a meter per minute. The most extensive lava flow emanated from a crater situated south of the top of the volcano, flowing in the direction towards Vatnafjoll, probably by rate of 6-7 m per hour.

Weather conditions and poor visibility have severely limited observations. At the present level of activity, lava flows and ash fall pose little danger to human settlement, as the farms closest to the advancing lava flows are about 10-15 km away. Ash from previous Hekla eruptions has commonly been very rich in fluoride and often been the cause of fluorosis in grazing animals. At this time of the year, however, most domestic animals are kept indoor, so fluorosis is not expected to become a problem during this eruption. Soluble floride (F-) which causes fluorosis, was measured in the freshly fallen ash as 800-900 mg F/kg. Snow melted from the ash contains about 2200 mg/l (ppm) fluoride.

On February 28 an eruption cloud is being deflected from Hekla towards the south by strong northerly winds, but otherwise the weather conditions have precluded direct observation of the eruption sites. The amplitude of the eruption tremor continues to decline at a slow rate, and the power of the eruption was obviously decreasing. By view from south at 3 PM two eruption clouds were seen, confirming that activity was still ongoing in two places in the volcano. The craters were not visible in the daylight but the more productive one could be located just south of the top of the volcano, producing three lava streams flowing down the south flanks of the volcano.The activity in the northern part of the volcano was declining during the day and couldn´t be located due to poor visibility. At 4:30 PM one lava flow had reached the Vatnafjoll mountains at Lambafell, 5 km south of the summit where it flowed slowly at approximately 2-3 m per hour. The thickness of the lava at the front was 8-10 m on average. This evening, strombolian activity in three separate craters at the southernmost part of the eruptive fissure was observed. No activity was observed in other areas of the volcano.

Early morning on February 29 activity was ongoing. Ash fall was reported this morning in Fljótshlíð 35-40 km south of the volcano, carried by strong northerly wind.During the morning activity seemed to be continously decreasing, and by around 8 AM all activity in the summit area of the volcano ceased. At 5 AM volcanic tremor had started to increase and this went on until 10-11 AM. In the afternoon, activity in the southernmost end of the fissure increased again, producing eruption cloud ascending a little above the top of Hekla. The ash was still carried southward. In the evening, darkness revealed the three strombolian craters observed the day before, producing lava flowing to southwest. This evening, people watching the lava at the north east flanks reported that the flow had stopped and they could walk on the lava.

Photos: Sigurjón Sindrason

On a reconnisance flight over Hekla between 11:00 and 12:30 on March 1 the southern part of the fissure was cloudless. However, the northern part was covered in clouds and thus not observable. Activity on the fissure that cuts up the SW slopes of Hekla is vigorous. Four main vents where observed and then three smaller vents. Activity in the craters was strombolian, with explosions on the interval of some 4 to 5 min. Effusion rate is similar as the day before although the explosion activity has diminished. Two main lava streams were observed. One going SW of the mountain and the seccond going straight to the south. The seven craters observed on the southern end fead the lava flow that goes towards SW. At the base of the fissure, that strikes up from the foot of Hekla and up to its shoulder, a large tumuli has developed. The lava streams out through an opening of the tumili and joins a stream comming from overflows in the uppermost craters. The lava that flows to the south is on the other hand fed by a crater close to the summit of Hekla, which has obviously been activated again. The lava field in the south has not advanced towards the south but 100 m max since Monday. However, it is growing towards the east at present.Little is known about the activity for the rest of the day due to bad weather conditions. At this time point lava covers approximately 17 km2 If estimated thickness of lavas is 6 m on average, calculated volume is 0.1 km3.

Bad weather hindered observation of Hekla since noon yesterday till midday March 2. Increased activity was observed in the top craters today. Constant steaming from the SW craters, compared to yesterdays observation there was much bigger steam clouds rising from the top craters. At night fall explosion were observed at about 1/2 hour interval. Glowing lava streams were also noted down the flank of the mountain.

There is a constant glow in the top craters, this is somehow different from what was observed on 29/2 when distinct explosion were observed and no glow in-between. The center and northern area could not be observed due to cloudy hat (the hekla). On 3 March we will attempt to approach the southern flank of Hekla and sample the lavastreams in the north east and south west, we will also do some GPS location of the lava fronts.

A group of scientists reached the SW lava flow at 13:00, 3 March. The thickness of the lava front was estimated about 10 m and the flow was advancing very slow, probably 1-2 m per day. The lava is spreading in a "cuvet" like depression. While tracing the lava to more westerly direction it was noted that at places the flow was spreading at much faster rate, estimated up to 1 m/hour.Following the lava flow along its westernmost side, the group reached its origin at the foot of the volcano. The lava was not flowing from any actual crater but welled up at the end of the erupting fissure. The lover most part of the fissure is sub terrain from the lowermost crater to an approximately 50 m high block of older welded rocks, resembling a horst formation. The lava was being expelled through an opening of the horst like structure.The flow rate of the lava at the origin was estimated 0,06 m/s, producing about 10 m3/s.The lava river can be traced for some distance by blue mist rising above it. The blue mist is caused by continuos degassing of the magma as it flows along the lava stream. Such blue mist was also observed farther east along the flank of the volcano. This is indicating that lava is still flowing from the crater close to the summit area. The craters in this region of the volcano feed the lava flow that flows southward to the Vatnafjoll area. Later in the evening this was confirmed by observers which reported that lava was still flowing slowly at Vatnafjoll. Explosive activity was ongoing in the uppermost crater of the SW-fissure. The activity was characterized by small explosions at 10 to 20 minutes interval, producing white steam clouds with only trace amount of ash.

Because of bad weather conditions no direct observations could be done on the eruption on 4 March.

5 March. The lava flow in the SW is still ongoing according to observations made in the afternoon. At sunset, red pulsing glow was observed from Selsund. The glow was located in the uppermost craters of the SW-fissure.

6 March. White small eruption clouds were observed to penetrated thin clouds covering the top of Hekla, as reported from Selsund this morning. Eruption tremor was declining in the afternoon. In a reconnaissance flight between 17:30 and 18:30 it was observed that steaming was vigorous along the whole eruptive fissure. It was also observed that all the lava flows had come to halt. The lava stream in the SW has stopped advancing and left behind a lava channel at the source. This indicates that feeding of the lava flow has stopped and the lava stream drained away to form the channel. Neither red glow nor explosive activity was observed along the crater row. The eruption has thus ceased for the time being. Minor tremors are still observed but they could be related to degassing of lava in the feeder dike.

7 March. No activity was observed, but small scale tremor was noticed on seismometers.

In the morning of 8 March at 8:44 the last eruptive tremor was detected on seismometers. By this definition of detection of tremor, and as no signs of eruptive production was observed since March 5, the eruption lasted from the afternoon 26 february to early morning March 8, or 11 days.

The lava production probably lasted for only 9 days. The lava covers approximately 18 km2 and preliminary estimate of lava production is 0.11 km3